Directional channel separation filters



May 31, 1960 P. G. MARIE. 2,939,093

DIRECTIONAL CHANNEL SEPARATION FILTERS Filed June 26, 1956 2 Sheets-Sheet 1 INVENTOR PIERRE G. MARIE y 1, 1960 P. e. MAlE 2,939,093

DIRECTIONAL CHANNEL SEPARATION FILTERS Filed June 26, 1956 2 Sheets-Sheet 2 INVENTOR PIERRE G. MARIE DIRECTIONAL CHANNEL SEPARATION FILTERS Pierre G. Mari, 16 Rue de Varize, Paris, France Filed June 26, 1956, Ser. No. 593,886

Claims priority, application France June 30, 1955 3 Claims. (Cl. 333-10) The present invention relates to directional channelseparation filters.

Resonant directional couplers have been constructed by the inventor which consist of two identical rectangular guides and of a circular cavity resonator which connects them together, such that a traveling wave of electromagnetic energy in the TE mode, which is propagated in a given direction in one of the rectangular guides and arrives in the junction region between the said rectangular guide and the cavity resonator, has its energy split into two parts. The first part (that which has frequencies outside the bandwidth of the cavity resonator) continues to progress in the rectangular guide to beyond the junction with the cavity resonator. The second part (that which has frequencies within the bandwith of the cavity resonator) of this energy is radiated in the circular resonator through the intermediary of a particular first coupling in the form of a circularly polarized wave which is constituted by two waves of the TE mode which are polarized perpendicularly and in quadrature. :circularly polarized wave radiates energy in the second The same rectangular guide through the intermediary of a second coupling which is similar to the first coupling and is propagated in the TE mode in a definite direction which depends upon the direction of propagation in the first rectangular guide.

In these couplers, the system of coupling between each such that wherein a is the dimension of the large side of the guide and A; is the wavelength in the rectangular guide.

It follows from this that these first kind directional :couplers are composed of first and second rectangular guide stubs identical with each other which are connected together by a circular cavity resonator, the axis of which is eccentric with respect to the plane of symmetry passing through the axes of the two rectangular guides. This eccentric structure may have disadvantages, especially from the point of View of construction.

Since the two bases of the circular cavity resonator are not entirely contained in the large faces of the rectangular guides,

it is especially necessary to arrange assymetric bottoms -on the parts of these bases which are outside the said large faces.

Applicant attempts to construct directional channel- 'separation filters by cascade-connecting a plurality of nited States Patent 0 resonant directional couplers of the above-mentioned first 2,939,093 Patented May 31, 1960 kind in such a way that the first rectangular guide stubs of all the couplers constitute a common rectangular main guide and by giving to the circular cavity resonators of the couplers resonance frequencies in the TE mode respectively equal to the median frequencies of the channels to be separated.

The directional channel separation filters thus constructed exhibit relatively poor voltage standing wave ratios of at least 1.20 which is a serious drawback for constructing directional multiplexing filters for the separation of a large number of frequency channels. This is due to the fact that the crossing slots being at a common location cannot be matched independently from each other to equalize their power coupling factors relatively to the transverse and longitudinal components of the magnetic field in the rectangular waveguide. And the same impossibility is met when the off-center aperture is a single circular one.

Furthermore, these directional channel-separation filters cannot be designed with standard size rectangular wave guides without the provision of flattening transitions in said wave guides at the location of couplings with the circular cavity resonators.

The object of the present invention is to construct a directional channel separation filter comprising a common rectangular main wave guide, a plurality of circular cavity resonators tuned onto different resonant frequencies and an equal plurality of frequency channel rectangular wave guide stubs, having a wide number of frequency channels lying in a wide bandwidth and exhibiting through the over-all bandwidth a voltage standing wave ratio quite lower than in the first kind directional channel-separation filters, say lower than 1.05. 7

Another object of the invention is to construct a directional channel-separation filter of the kind concerned in which the condition of complete rejection between the two arms of the main guide on one side and the other of patible with standard size rectangular waveguide construction.

According to the invention, the systems of slots between the circular cavity resonator and each rectangular guide of the directional coupler, the axes of which meet each other or are very near each other, comprise a slot which is longitudinal with respect to the rectangular guide and located on one side of'the axis of the said guide and a transverse slot, which is located on the other side of the axis, the distances between this axis and the centers of the slots being fixed by relationships which will be set forth hereinafter.

Furthermore a small metallic plate is inserted in each of the rectangular guides, parallel to the wide side of the guide, and facing the portion of wall comprised between the longitudinal slot and the edge of said wide side.

The invention will be better understood on reading the detailed description which will now be given and on examining the accompanying drawings, of which Fig. 1 represents a directional coupler of the prior art;

Fig. 2 represents a directional coupler used in the invention;

Fig.3 is a diagram of the junction between a rectangular guide of the coupler and the circular cavity resonator explaining the fixing of the geometrical position of the slots;

Fig. 4 is a directional channel-separation filter using a plurality of couplers of the type shown in Fig. 2.; and

Figs. 5a and 5b are diagrams of waveforms useful for explaining the operation of the coupler.

In Fig. .1, which represents a directional coupler of the prior art, 1 is the first rectangular guide, 2 is the second rectangular guide, 9 is the circular guide, 3 and 4 are the systems of slots, and 7 are the mouths of the guide 1, and 6 and 8 are the mouths of the guide 2. The centers of the slots 3 and 4 are at a distance x from the edge of the rectangularguides given by the above Equation 1 and'are on the axis' 10 of the circular guide. 7

The dimensions of the rectangular guides area and b (a b) and the diameter of the circular guide is D. It is seenin Fig. 1 that the axis 10 does not meet the axes 11 and 12 of the large sides of the rectangular g'uides which are facing one another and that the parts 13 and 14 of the bases of the circularguide extend beyond the edge of the large facesof the rectangular guides. Naturally, these parts 13 :and 14 should be closed by metallic bottoms. V V i w r Further, as will be seen hereinafter, the conditioin-of complete rejection between the arms of the common rectangular main waveguide on one :side and the other of a cavity resonator pertaining toa .given channel in the frequency bandwidth of said channel occurs for ratios be tween the small and large side widths of said wave guide which are not those of standard size waveguides. The sameistrue for the complete rejectioncondition between the two arms of a channel-rectangular waveguide.

Referringto Fig; 2, which represents a-couplerused in the inventiomthe said coupler comprises two-rectangular guides 15 and 16 and a circularguide 18, the axis 19 of which meets the axes 20 and 21 of the large sides of the rectangular .guides. The systems of slots each comprise two slots '22 and 23, onebeing longitudinal-and theother transverse with respect to the rectangular guides and situated respectively on one side and the-otherof the axes 20 and 21 of the large sides of these guides. As shown in'Fig. 2, slots 23 must be integrally on one side only of the longitudinal axes 20, 21 of the corresponding rectangular guides and must not cross the same. 7

Small metallic plates 24, the very important function of which will be explained hereinafter, are arranged in the rectangular guides parallel tothe large side and facing the portion of wall comprised between the edge of the large side and the longitudinal slots 22.

In Fig. 3, the system of slots 22-23 is represented with reference to the axis 0y parallel to the axis 20 of the guide .15 and to the axis Ox which is perpendicular to the foregoing axis. The point 0 is the projectionof the axis 19. g

If theabscissa of the side 25 of therectangular guide 15 is denoted by x,;, the abscissa of the side 26 of the same guide is denoted by ax thedistance of 0 to the centerofthe slot 22 is denoted by x and the distance-of O to the centerof the slot 23 is denoted by x it may be shown. that, amongthe six quantities x x x ,-q, b and D, there are threerelationshipsand that one is, consequently, allowed to impose three arbitrary conditions;

These relationships may be developed-by the following method: Let 2 cot u bethe relativessusceptance ofthe slots, i.e.-the-ratio ofthe susceptances of theslots respectively in therectangular guide and in the circular guide by the admittances of said guides (see Marcuvitz, Waveguide Handbook, Radiation laboratory Series, vol. '10, page 365, McGraw-Hill Book Company, Inc., New York, 1951). It is known that tan a is proportional, at'a given frequency, to the square of the component of the magnetic field alongthe slot of a unit power wave. Let us denote by H H H respectively the maximum transverse magnetic field in the rectangular guide, the'maximum longitudinal magnetic field in the rectangular guide and the maximum transverse magnetic field in the circular guide for a unit power wave (TE inthe rectangular guide and TE -in the circular guide).

' The magnetic field alongthe slot,22is:'

therectangularguide and, the circular guide:

5 HTR sin in the rectangular guide and, in the circular guide:

The traveling wave in one of the slot coupling systems, say the couplingsystem between guides '15 and 18 may be considered as the superposition of a double set of standing waves, applied to the'two mouths of guide 15 and to r the mouthof guide 18 assumed to be disconnected from guide 16 and to have its top open, and respectivelysymmetrical and antisymmetrical with respect to the plane xOz. The symmetrical waves are shown in Fig. 5a and the antisymmetrical waves in'Fig. 5b. As for the symmetrical standing wave in guide 18, it is polariz edin the xOzplane, whereas the antisymmetrical wave in the same guide ispolarized in the yOz plane. As the slots have a susceptance angle u, the nodesof the standing waves are displacedbyan angular distance ufrom "the" location-they 25 wouldhave if the slots-were omitted. Now, let us associate thestandingwave-pattern A'o'f Fig. 5a and the standing I wave pattern B of Fig. 5b and form the pattern A-I-jB (by .superposing the pattern A with the pattern-B in phase quadrature with the first);- there is obtained a traveling wave in the'rectangular-guide a-circularlypolarized standing wave in the circular guide;

As already stated, tan 1: is proportional to thesquare of the component'of the magnetic field along the slot. Then, by equaling the four expressions of tan u, we may the fact that, in the rectangular guide, the variation of reactive energy in the vicinity of the slots islproduced by thetwo waves applied tothe ends of said guide.

It maybe noticed that, whereas the three first expressions are quite equal with each other, it is notexactly true for the fourth. In fact in the three firstexpressions, the magnetic field which is "concerned is the transverse one and the scattering by the slots of the pattern of said field normally produces a longitudinal displacement of the nodes of the standing waves in the rectangular guide. In the case of the symmetrical standing waves intherectangular guide, it is the longitudinal component of the magnetic field which is concerned and has its pattern scattered by the slots. To the scattering of said field there would rather correspond a variation of the widthfqf the rectangular guidethan a longitudinal displacement of .the nodes of the standing waves. Nevertheless, 'the :fourth expression will be retained, but, ash is only roughly equal to the three other, the coupling system cannotsuitably operate without supplementary means which will be disclosed hereinafter.

From Equations 2, one may derive:

asaaoes in which 7t is the wave length in free space of the wave which travels in the guides.

The first of the conditions which may be applied to the six quantities x x x a, b, D, which favours the construction and the operation of the coupler, is to make the cut-oif wavelength k for the wave TB in the rectangular guide nearly that of the cut-ofi wavelength A for the wave T13 in the circular guide, which is expressed by It is shown by the third Equation 3 that, when condition (4) is satisfied, the ratio b/a for a correct operation of the coupler is independent of the frequency.

As second and third conditions, there may be taken under a first hypothesis that is to say, there may be imposed on the rectangular slots the condition of being concentric and, consequently, of forming a cross. There is then obtained The system of slots thus determined is that of a coupler of the prior art. As will be seen hereinafter in an example, the second Formula 6 gives, for the ratio b/a, lower values than those allowed in the standard rectangular guides and it is necessary to diminish b by transition 24 and 35 (Fig. 1) at the place of the slots; this constitutes a troublesome structural condition.

Under a second hypothesis, there may be imposed, as

the second condition,

271' cos 2 b 1 a 1 94 '71 (8) 7T2: cos a Formulae 8 enable x x and b to be determined for a given value of the ratio Let us suppose that it is desired to construct a directional coupler which allows a frequency band to pass which comprises between (375010) mc./s. and (3750+l0) rnc./s. corresponding to a mean wavelength in the unbounded space which is equal to )\=80.6 mms. For this wavelength, there is chosen, as usual, a rectangular guide having a large side a such that a =57 mms.

in V5 The dimensions of the coupling system are then thefollowing under the first hypothesis:

D=66.8 mms. x =l4.25 mms. b=14.7 mms.

Under the second hypothesis, which is that of the present invention, the last of the Formulae 8 is, when The coupling system then has the following dimensions:

D=66.8 mms. x =18.2 mms. x =10.3 mms. b=26.6 mms.

It is observed that, under the first hypothesis whilst, under the second hypothesis,

which value is acceptable because, in the standard guides, the ratio b/a is generally equal to about 0.44.

For theoretical reasons, above summarily explained, and in order to increase the transmission band of the directional couplers, it is necessary to arrange thin metallic plates parallel to the large sides of the rectangular guides and facing the portion of wall comprised between the edge of the large side and the longitudinal slots. The positioning of these plates 24 is determined by trial and error. In the case of the foregoing example in which b=26.6 mms., the plates are placed 3 mms. below the slots 22, i.e. at about an eighth of the height b, and they extend inward as far as the axis of the slot 22, that is to say that they have a width which is very nearly equal to Fig. 4 represents a directional channel-separation filter according to the invention.

The energy arrives from an aerial (not shown) through the main guide 32 which is terminated by a dissipative load element in the form of, for example, an absorbent plate 33. This energy is constituted by a wideband wave comprising two partial bands or channels.

Two circular cavity resonators 29 and 29' are coupled to the said main guide 32 by systems of slots 2223 and 22'23' of the type shown in Fig. 3. The circular resonators 29 and 29' are coupled to rectangular guides 27 and 27 by systems of slots which are identical with the foregoing. The guides 27 and 27' are terminated, at one of their mouths, by absorbent plates 28 and 28'. The energy corresponding to the first channel is obtained at the mouth 30 of the guide 27 and the energy corresponding to the second channel is obtained at the mouth: 30 of the guide 27'. Screws 31 and 31, four in number, in a diametral plane of the circular cavity resonators 29 and 29' provide tuning means for tuning the same,

esons to the central frequencies of the first and second channels respectively. I a a What I claim is: V v .r

' 1. A directional channel-separation filter adapted to separate a plurality of frequency channels contained in a largeover-all bandwidth electromagnetic energy comprising a commontrectangular main waveguide adapted to convey in the ,TE mode'said ele.c"tromflgncticjenergy,

a plurality, equal to the'plurality of frequency channels, of circular cavity resonators respectively tuned onto the mean frequencies of the frequency channels and adapted to support in the TE mode the electromagnetic energy of the corresponding channels, coupled to said rectangular main waveguide along the large side thereof and having axes meeting with the longitudinal axis of the rectangular main waveguide, a first equal plurality of slot-and-plate coupling systems coupling said rectangular main waveguide to said circular cavity resonators, an equal plurality of channel rectangular waveguides having the same cross-section than the rectangular main waveguide, respectively coupled along their large sides to said circular cavity resonators and having'longitudinal axes respectively meeting with the circular cavity resonator axes, a second equalplurality of slot-and-plate coupling systems coupling said manner rectangular waveguides to said circularcavity resonators, each slot-andplate coupling system of the first and second pluralities comprising ,a first slot longitudinal with respect to the rectangular waveguide to which it pertains, on one side of the longitudinal axis of said rectangular waveguide and spaced apart from said rectangular waveguide axis by a distance x and orthoradial with respect to the circular cavity resonator to which it pertains, a second slot transverse with respect to the rectangular waveguide to which it pertains, on the other side of the longitudinal axis of said rectangular waveguide and having a center spaced apart from said rectangular waveguide axis by a distance x and radial with respect to the circularicavity resonator to which it pertains, the centers of said 'slo'ts' being in a plane normal to the axis ofrthe rectangular waveguide and diametrical with respect to the circular cavity resonator and the quantities x and x being related by the relationship x =1.76x and a metallic plate located inside the rectangular waveguide and beneath the first slot only at a distance from said slot substantially equal to the eighth of the small side of the rectangular waveguide, an absorbent load at one end of themetangular main waveguide and absorbent loads at one of the ends of the channel rectangular Waveguides. 1 t

2. A directional channel-separation filter adapted to separate a plurality of frequency channels contained in a large over-all bandwidth electromagnetic energy comprising a common rectangular main waveguide adapted to convey in the TE mode said electromagnetic energy, a plurality, equal to the plurality of frequency channels, of circular cavity resonators respectively. tuned onto the mean frequencies of the frequency channels and adapted to support in-the TE mode the electromagnetic energy of the corresponding channels, coupled to said rectangular main waveguide along the large side thereof and having axes meeting with the longitudinal axis of the rectangular main waveguide, a first equal plurality of slot-and-plate coupling systems coupling said rectangular main waveguide to said circular cavity resonators, an equal plurality of channel rectangular waveguides having the same cross-section than the rectangular main wave guide, respectively coupled along their large sides to said circular cavity resonators and having longitudinal axes respectively meeting with the circular cavity resonator axes, a second equal plurality of slot-and-plate coupling systems coupling said channel rectangular waveguides to said circular cavity resonators, each' slotand-plate coupling systems of the first and 'second'pluralities comprising a first slot longitudinal with respect to the rectangular waveguide to which it-pertains, on one side of the longitudinal axis of said rectangular waveguide and spaced apart from said rectangularwaveguide axis by a distance x;, and orthoradial with respect to the circular cavity resonatorto' whichit pertains; asecond slot transverse with respect totlie rectangular waveguide to which it pertains, on the other side of the longitudinal axis of said rectangular waveguide and having "a center spaced apart from said rectangular waveguide axis by a'dist'ance x and radial with respect to the circular cavity resonator to which it pertains the centers-of said slots being in a plane normal to the axis of the'rectangular waveguide and diametrical With respect to the circular cavity resonator and the quantities x and x being related by the relationships:

ee -76a k being the mid-wavelength of the over-all bandwidth and a the great sidedirnensions of the rectangular guide, and a metallic plate located inside the rectangular waveguide and beneath the first slot only at a distance from said slot substantially equal to the eighth of the small side of the rectangular waveguide, an absorbent load at one end of the rectangular main-guide and absorbent loads at one ofthe ends of the channel rectangular Waveguides. i 7

3. A directional channel separation filter adapted-to separate a plurality of frequency channels contained in a large over-all bandwidth electromagnetic energy comprising a common rectangular main waveguide, having a large side dimension equal to a, adapted to convey in the TE mode said electromagnetic energy, a plurality, equal to the plurality of frequency vchannels, of circular cavity resonators all having a diameter equal to 1.172 a, respectively tuned onto the mean frequencies'of the frequency channels and adapted to support in the TE mode the electromagnetic energy of the corresponding channel, coupled to said rectangular main Waveguide along the large side thereof and having axes meeting with the longitudinal axis of the rectangular main waveguide, a first equal plurality of slotand-plate coupling systems coupling said rectangular main wave-guide'to said circular cavity resonators, an equal plurality of channel rectangular wave guides having the same cross-section than the rectangular main wave-guide, respectively coupled along their large sides to said circular cavity resonators and having longitudinal axes respectively meeting with the circular cavity resonator axes, a second equal plurality of slot-and-plate coupling systems coupling said channel rectangular waveguides to said circular cavity resonators, each slot-andplate coupling system of the first and second pluralities comprising a first slot longitudinal with respect to the rectangular waveguide to which it pertains, on one side of the longitudinal axis of said rectangular waveguide and spaced apart from said rectangular waveguide axis by a distance x and orthoradial with respect to the circular cavity resonator to which it pertains, a second slot transverse with respect to the rectangular waveguide to which it pertains, on the other side of the longitudinal axis of said rectangular waveguide and having a center spaced apart from said rectangular waveguide axisby a distance x and radial with respect to the circular cavity resonator to whichit pertains, the centers of said slots being in a plane normal to the axis of 'the'rectangular waveguide and diametrical with respect to the circular cavity resonator and the quantities x and x being related by the relationship x =l.76x and a metallic plate located inside the rectangular Waveguide and beneath the first slot only at a distance from said slot substantially equal to 9 the eighth of the small side of the rectangular waveguide, an absorbent lead at one end of the rectangular main waveguide and absorbent loads at one of the ends of the channel rectangular waveguides.

References Cited in the file of this patent UNITED STATES PATENTS 2,283,935 King May 26, 1942 10 Preston Feb. 5, 1952 Pierce Jan. 27, 1953 Cohn Nov. 8, 1955 Edwards Sept. 10, 1957 Cohn Sept. 9, 1958 

